Abrasive grain and manufacture for the same

ABSTRACT

An abrasive grain for use as a grinding material comprising alpha-alumina crystal particles substantially smaller than 0.5 microns, solidly dissolved with at least one of Ti, Mn, V, Ga, Zn and Rh, and with an a-axis length of the hexagonal unit cell of the alpha-Al 2  O 3  from 4.75892 to 4.76340 angstroms when measured by a powder X-ray diffraction method, and a density of more than 90% of the theoretical value, and an abrasive grain for a grinding material, comprising alpha-alumina crystal particles substantially less than 0.5 microns, solidly dissolved with at least one of Mg, Ni and Co, and with an a-axis length of the hexagonal unit cell of the alpha-Al 2  O 3  from 4.75930 to 4.76340 Å when measured by a powder X-ray diffraction method, and a density of more than 90% of the theoretical value, and further, a grinding wheel and coated cloth manufactured from these abrasive grains.

This is a divisional of application Ser. No. 07/845,828 filed Mar. 6,1992, now U.S. Pat. No. 5,192,339, which is a continuation ofapplication Ser. No. 07/474,041 filed Apr. 25, 1990 now abandoned.

TECHNICAL FIELD

The present invention relates to an abrasive grain to be used as adurable polycrystalline sintered ceramic grinding material and to itsmethod of manufacture. The grain contains alumina as a base material andis made by means of an improved sol/gel process.

BACKGROUND ART

A manufacturing method of an abrasive polycrystalline sintered ceramicgrain for grinding, using high-density alumina (Al₂ O₃) as a basematerial and a sol/gel manufacturing process, is well-known. Patentapplication JPA 56-32369 (corresponding to U.S. Pat. No. 4,518,397 andEP-A-24099) describes that an alumina hydrate may be gelled togetherwith a precursor comprising at least one kind of reforming component,then dried and sintered. The reforming component used in this caseincludes oxides of Co, Hf, Mg, Ni, Zn and Zr. Patent applicationJP-A-60-231462 (corresponding to U.S. Pat. No. 4,623,364 and EP-A-152768) describes a sol/gel method for accelerating the manufacture ofhigh-density alumina by adding alpha-alumina seed crystals, which may beadded to the sol as a crystal growth control agent comprising oxides ofSi, Cr, Mg and Zr.

Patent application JP-A-61-254685 (corresponding to U.S. Pat. No.4,744,802 and EP-A-200487) states a method for adding alpha-alumina,alpha-ferric oxide, or their precursors into a sol as a nucleatingagent, and includes a statement that the gel contains precursors ofoxides of Mg, Zn, Co, Ni, Zr, Hf, Cr and Ti.

Although producing a sintered alumina abrasive grain by a sol/gelprocess yields strong fine abrasive polycrystal grains, the crystal sizeof the grain is coarse and does not have uniform size without addition.Therefore, the grain is improved by adding reforming components, such asMgO and ZrO₂, into the alumina sol material, as described above.However, because low-hardness substances like spinel, are formed in thegrain boundary of the polycrystalline abrasive grain, the averagehardness and strength of the abrasive grain could be improved evenfurther. Although the hardness of the grain has been addressed to someextent by these methods, further improvements are both possible anddesirable.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides a strong abrasive grain made by combininga micro-crystallizing technology that uses a sol/gel process and acrystal strengthening technology that uses solid solutions.

Upon request from the grinding industry, the applicants of the presentinvention have developed a method wherein the resulting crystals areboth finer and stronger. According to this method, no layer softer thanalpha-Al₂ O₃ is formed in the grain boundary.

Therefore, the present invention provides an abrasive grain, for use asa grinding material, that comprises alpha-alumina crystal particles witha crystal size substantially less than 0.5 microns; the grain is a solidsolution with at least one of Ti, Mn, V, Ga, Zn and Rh; the measurementof the "a" axis length of the hexagonal unit cell of alpha-Al₂ O₃ in thegrain is 4.75892 to 4.76340 angstroms (Å) when measured by powder x-raydiffraction; and the density of the grain is more than 90% of thetheoretical value.

Furthermore, the present invention provides an abrasive grain, for useas a grinding material, that comprises alpha-alumina crystal particleswith a crystal size substantially less than 0.5 microns; the grainconsists of a solid-solution containing at least one of Mg, Ni and Co;the value of the "a" axis length of the hexagonal unit cell of Alpha-Al₂O₃ in the grain is 4.75930 to 4.76340 angstroms when measured by powderx-ray diffraction; and its density is more than 90% of the theoreticalvalue.

In addition, the present invention provides a method for manufacturingan abrasive grain for use as an alpha-Al₂ O₃ grinding material; theabrasive grain added before the sol is gelled comprises at least one ofthe following materials, all of which have an alpha-alumina structureand a particle size substantially less than 2 microns Ti₂ O₃, MgO·TiO₂,FeO·TiO₂, NiO·TiO₂, CoO·TiO₂, MnO·TiO₂, ZnO·TiO₂, V₂ O, Ga₂ O₃ and R₂O₃, or an alpha-Al₂ O₃ solid solution in combination with theseelements. The material chosen is added into the alumina sol, the sol isgelled and then sintered at 1,000° C. to 1,400° C.

Furthermore, the present invention provides a grinding wheel and coatedabrasive paper which contain the above-mentioned abrasive grain.

Ti₂ O₃, if used, may be added into the alumina sol as a quadrivalenttitanium compound (TiO₂, for example), which is gelled and then sinteredat 1,000° C. to 1,400° C. in a reducing atmosphere. This is possiblebecause TiO₂ is reduced during the sintering to form Ti₂ O₃. In anilmenite system, a titanium compound (Ti(OH)₄, for example) and acompound of other constituting metals (Ni(NO₃)₂, for example) may beadded at a molar equivalent per the titanium and the other metal M(nickel, for example). In this case, also, MO-TiO₂ is formed duringsintering, though it does not constitute 100% of the product.

If particles having alpha-alumina structure are added into the aluminasol as seeds, the energy needed to form alpha-alumina crystal nuclei isspared, and therefore the alpha-alumina crystals can grow at a reducedtemperature. As a result, the growth of crystals having undesirablestructure, which often occurs at high temperatures, can be avoided. Thisconserves energy for the crystals to grow epitaxially on the seeds (theadded particles) rather than causing the growth of new alpha-aluminacrystal nuclei. Accordingly, alpha-alumina crystal particles are moredifficult to develop, a uniform particle size is obtained, and theabrasive grain strength is improved. Energy conservation is greatestwhen using a crystal identical to the deposited crystal. Less energy isconserved when using a crystal having the same structure, and even lessenergy is conserved when using a crystal having like structure. Crystaldeposition and growth is optimized using seed particles having the samealpha-alumina structure as pure alpha-alumina (Al₂ O₃)

However, the applicants of the present invention have discovered that,in order to produce an abrasive grain with stronger alpha-aluminacrystals, it is better to add into the alumina sol particles having analpha-alumina structure and the like, containing the elements that canbe solidly dissolved into the alpha-alumina, rather than to add purealumina as the seed.

The particles having an alpha-alumina structure include Ti₂ O₃,MgO·TiO₂, FeO·TiO₂, NiO·TiO₂, CoO·TiO₂, MnO·TiO₂, ZnO·TiO₂, V₂ O₃, Ga₂O₃ and R₂ O₃, or an alpha-Al₂ O₃ solid solution with these compounds.The metal ion addition described above does not produce a remarkableimprovement in a solid solution. Other oxides of these elements do notform a true solid solution. The amount of the particles added to thesolid solution is preferably 0.005 to 2.23 mol %, and more preferably inthe range of 0.01 to 1.15 mol %. If the added amount is less than 0.005mol %, an enhancement in the alpha-alumina crystal strength cannot beexpected. If the amount is more than 2.23 mol %, solid solubility of thealpha-alumina crystals is exceeded, causing local crystal deposition inthe grain boundary of the alpha-alumina crystals, which causes theformation of soft layers in the grain boundary, and results in anoverall decrease in hardness and strength of the abrasive grain. Ifseeds are added to an extent that enhances the grain strength and doesnot reduce it, the abrasive grain made according to the presentinvention has an a-axis length of the alpha-Al₂ O₃ hexagonal unit cellfrom 4.75892 to 4.76340 Å when measured by powder x-ray diffraction.

The smaller the crystal size in an alumina sintered abrasive grain, thehigher its grinding performance. Generally, however, if a lowtemperature is used in sintering, the crystal size is small, but notsufficiently dense. The smallness in crystal size alone is notsufficient to improve the grinding properties. The sintering densitymust also be raised to at least 90% of the theoretical value. Thepresent invention provides such an alumina sintered abrasive grain.

If the density if raised, the sintered alumina abrasive grain becomesstronger. If the crystal size is reduced, the grain withstands therequirements of heavy-duty grinding.

Furthermore, the crystal size should be substantially less than 0.5microns. Therefore, the size of the seed to be added into the aluminasol must be less than 2 microns. If the seed crystals are small, lessmaterial is required, and the characteristics of the abrasive grainproduct are improved.

In the present invention, at least one of Ti₂ O₃, MgO·TiO₂, FeO·TiO₂,NiO·TiO₂, CoO·TiO₂, MnO·TiO₂, ZnO·TiO₂, V₂ O₃, Ga₂ O₃, and R₂ O₃, or analpha-Al₂ O₃ particle that can be solidly dissolved with the elementscontained in these compounds is added into an alumina sol. If compoundsother than aluminum are added into an alumina sol in an ionic state, asdescribed in previous inventions and in Japanese patent applicationJP-A-57-207672, these elements remain in the grain boundaries of thealumina crystals even after drying and sintering. There, they suppressthe formation of unusually large crystals, and they do not make thecrystals grow epitaxially on the seeds as in the present invention, andthey do not make sintered grains comprising small crystals in whichthese ions are solidly dissolved. In addition, titanium ion forms TiO₂when oxidized in air, and this is deposited at the grain boundaries ofthe alumina crystals.

The Ti₂ O₃ used in the present invention, as is well known, is formedonly in a non-oxidizing atmosphere or in a vacuum. Therefore, unless itis added in the form of Ti₂ O₃, it will not behave as intended by thepresent invention even if the titanium ion is added into the sol, norwill the titanium form a solid solution if it is not sintered in areducing atmosphere as described above. If Ti₂ O₃, a meta-stablecompound of the same form as alpha-Al₂ O₃, is oxidized, it forms astable TiO₂ and is deposited at the crystal grain boundary when addedinto an alumina sol even when in the form of a fine grain, not tomention in the form of an ion. In the present invention, the Ti₂ O₃ seedis covered by the alumina sol, whereby the compound will not be exposedto an oxidizing atmosphere even in an ordinary process of making aluminasintered abrasive grain. In this way it remains as Ti₂ O₃, without beingtransformed into TiO₂, and the alumina is then deposited on its surface.

V₂ O₃ is formed by reducing V₂ O₅ by hydrogen or carbon, and undergoesthe same reactions and produces the same phenomena as in the case of theabove Ti₂ O₃, forming a meta-stable compound.

MgO·TiO₂, FeO·TiO₂, NiO·TiO₂, CoO·TiO₂, MnO·TiO₂, and ZnO·TiO₂ have anilmenite structure. FeO·TiO₂, in particular, is a natural mineral foundin large quantities. These compounds can be obtained when the hydroxidesor carbonates of each of the metals are reacted with TiO₂. For example,when MgO·TiO₂, is put into an alumina sol in the form of MgO and TiO₂independently, MgO·Al₂ O₃ (spinel) is generated preferentially asdescribed above, unless it is added in a well dispersible form (forexample, Mg(OH)₂ and Ti(OH)₄). Therefore, the above metals must be addedin the form of MgO·TiO₂ (ilmenite compound), for example, as mentionedabove.

Measurement accuracy for the lattice constant of alpha-Al₂ O₃, one ofthe components of the present invention, is described as follows. Thelattice interval, d, of a (330) plane is measured, and the spacing issextupled to obtain the a-axis length. The diffraction angle of a (330)plane, 2-theta, is 152.4 degrees when Cuk.sub.α is used as the mostcommon x-ray. In the high angle of this extent, K.sub.α1 and K₂ arecompletely separated, and the value of 2-theta can be measured up to0.001° C. by a goniometer. Therefore, lattice interval d can be measuredto six significant figures. It is noted that the a-axis length of purealpha-Al₂ O₃, which is not in the form of a solid solution, is 4.75890angstroms.

The grinding wheel, according to the present invention, is made of theabove-described abrasive grains and a bonding material, such as avitrified bond, metal bond, or resin bond. A grinding wheel that uses avitrified bond is a preferred application of the present invention.

The bond used in the vitrified grinding wheel, generally called a frit,is a carefully proportioned mixture of feldspar, pottery stone, boraxand clay and its constituents include SiO₂, B₂ O₃, Al₂ O₃, Fe₂ O₃, CaO,MgO, Na₂ O, and K₂ O. Such a bond, with a small amount of a starch likedextrin added, is mixed with abrasive grains, formed in a press, driedand fired to produce a vitrified grinding wheel. When the presentinvention is employed, the firing is perferably performed at 950° C. to1150° C. so that the crystals of the abrasive grain do not becomecoarser.

A coated abrasive paper is made of a base material bonded with theabrasive grain using a bond such as a phenol resin bond which is mostoften used for its excellent abrasion and water resistance. The bondingmay be easier if the bond is mixed with resorcinol or its derivatives.Paper, cloth, non-woven cloth and the like are utilized as basematerials.

A more detailed explanation of the present invention is set forth belowwith reference to examples. The present invention is not limited tothese examples.

EXAMPLE 1

First, titanium metal powder was mixed with TiO₂ and formed intopellets. The pellets were sintered in a vacuum for one hour at 1400° C.,pulverized, and the particles classified to obtain fine Ti₂ O₃ particlesfor seed having a maximum particle size of 0.5 microns and an averagesize of 0.2 microns.

Next, 0.03 g of seed was suspended in 20 ml of water acidified to pH 3by nitric acid, and 4 g of commercial-grade boehmite was added to thissuspension to produce a sol. While the sol was maintained atapproximately 40° C. for about 24 hours, it was stirred by a magneticstirrer with a heater to produce a gel. The gel was then dried for 3days at 100° C. This dry gel was coarsely pulverized to produceparticles of less than 1 mm. The particles were places in an aluminacrucible and subjected to calcination in a muffle furnace at 750° C. for120 minutes under a flow of hydrogen, followed by full sintering at1250° C. for 100 minutes. After being allowed to cool naturally, asample particle was divided into two pieces, and the exposed face wasexamined under a scanning electron microscope. Observation revealed thatthe crystals in the particles had a well-defined size of 0.4 microns.Xylene immersion showed the specific gravity of this sample to be 3.96.

The sample particles were then further pulverized to about 10 microns tomeasure the a-axis length of the hexagonal unit cell by means of apowder x-ray diffraction process. The particles had an a-axis length of4.76190 angstroms, larger by 3.0×10⁻³ angstroms than pure alpha-Al₂ O₃.

Next, the sample particle was embedded into a resin, polished to mirrorface by grinding, and given an indentation with a load of 500 g using amicro Vickers indenter. The sample developed very few lateral cracks orPalmgvist cracks which can be seen in pure alpha-Al₂ O₃ sinteredparticles (this is explained in Ceramics Japan, 20 (1), 12 (1985).) Thismeans that the sample particle has a much greater strength than purealpha-Al₂ O₃.

EXAMPLE 2

A sample particle was prepared by the same method as in Example 1,except that 0.6 g of 0.2 micron seed was used, and the seed contained2.8 wt. % Ti₂ O₃ in an alpha-Al₂ O₃ solid solution. The a-axis length ofthis sample was 4.75930 angstroms, with very few lateral and Palmgvistcracks.

EXAMPLE 3

Mgo and TiO₂ were reacted to make MgO·TiO₂, which was pulverized, andthe resulting particles classified to obtain MgO·TiO₂ particles for seedhaving an average size of 0.2 microns. Using this seed, a sampleparticle was prepared in the same manner as in Example 1. The a-axislength of this sample was 4.76190 angstroms. This sample also developedvery few lateral and Palmgvist cracks under an indentation test using amicro Vickers indenter.

EXAMPLE 4

875 g of commercial-grade nickel carbonate, NiCO₃ ·2Ni(OH)₂ ·4H₂ O. and500 g of anatase titanium oxide were put into a polyethylene containertogether with polyurethane balls of 20 mm in diameter as a stirringmedium. These ingredients were mixed in a rotating pot mill rack, thenheated to 1250° C. for two hours in a muffle furnace. About 950 g of areactant were removed and observed under an x-ray diffractometer, bywhich NiO·TiO having a similar diffraction pattern with alpha-Al₂ O₃ anda small amount of NiO were identified. The obtained nickel titanate waspulverized in an iron pot mill for four days. The pulverized materialwas sufficiently adhesive to line the inside wall of the iron pot milland the surface of the iron balls of its own accord. Consequently, therewas very little contamination with iron. However, the material waswashed with excess hydrochloric acid for the sake of caution.

Next, the above suspension was decanted to remove the hydrochloric acid,and the material was repeatedly washed with water. After severalwashings, when the top of the liquid was hardly free of turbidity, thespecific surface area of the suspended solids was 80 m₂ /g. The ironcontamination was 0.05% by weight after washing.

To investigate the efficacy of the slurry of the NiO·TiO₂ fine powderthus obtained, the slurry was added to 8 kg of commercial-grade boehmite(SB Alumina made by Condea) so as to form a mixture which contained 1%by weight of NiO·TiO₂. The total volume of the mixture was 44 liters ofwater (including the slurry) and 500 ml of 67.5% nitric acid to make analumina sol. The sol was stirred for about two hours, then heated at 80°C. for 16 hours to produce a gel. The gel was desiccated by maintainingthe gel at 120° C. for 7 hours, ground, and passed through a sieve toobtain particles of about 500 to 300 microns. Then, crystallized waterwas removed from the particles by heating at 600° C. for two hours in amuffle furnace, and the particles were put into a rotary kiln in whichthe temperature was raised from room temperature to 1300° C. in 1minute. The sample was kept at 1300° C. for one minute, then treated for10 hours at 1100° C. The density of the particles at this time was 3.95,or 99% of the theoretical value. The Vickers hardness under a load of500 g was 2260 kg/mm².

In addition, to observe the crystal size of particles, the sample wasimmersed in a 90° C. saturated solution of borax (Na₂ B₄ O₇ ·10·H₂ O)and then etched at 900° C. for 30 minutes. After cooling, the sample waswashed with dilute hydrochloric acid to remove the glass layer formed onthe surface, and subjected to an SEM observation. SEM photographicobservation at a magnification of 20,000 showed that the crystal size ofparticles ranged from 0.15 to 0.40 microns, with an average particlesize of 0.23 microns. No particles larger than 0.5 micron were found.

The particle size after sintering ranged from 350 to 250 microns, whichcorresponds to abrasive powder #60 in JIS R6001-1973.

EXAMPLE 5

One kilogram of sandy ilmenite as a natural mineral and 1 liter of waterwere put into an iron pot mill and pulverized for three days using a wetprocess. Applying the same process as in Embodiment 4 produced a slurrywith a solid content of 6.1 mg/ml and a solid specific surface area of51 m₂ /g. Sintered particles were obtained by applying the same processas in Example 4, except that the slurry was added to the boehmite at0.5% by weight (Dry Base). Measurements showed that the micro Vickershardness was 2210 kg/mm², and the crystal size was 0.50 microns.

EXAMPLE 6

Except for using 288 g of commercial-grade manganese carbonate (MnCO₃)and 200 g of anatase-type titanium oxide as raw materials, the sameprocess as in Example 4 was applied to obtain an MnO·TiO₂ slurry with asolid content of 43 mg/ml and solid specific surface area of 102 m² /g.Sintered particles were obtained by the same processes as in Example 4.The micro Vickers hardness of the product was 2120 kg/mm².

EXAMPLE 7

A small amount of TiCl₄ was dissolved into water to obtain a suspensionof Ti(OH)₄. After by-product HC1 was removed using an anion exchangeresin, the suspension was added in place of the Ti₂ O₃ used in Example1, in an amount equivalent to 0.03 g Ti₂ O₃ to produce a sol.Thereafter, using the same process as in Example 1, the gel was heatedin a hydrogen atmosphere in a furnace, in which the temperature wasraised to 800° C. in three hours and from 800° C. to 1300° C. in 17minutes.

The gel was maintained at 1300° C. for five minutes. Then the heatingwas stopped and the sample was allowed to cool. The obtained aluminaparticles had approximately the same characteristics as those obtainedin Example 1.

EXAMPLE 8

A small amount of TiCl₄ and Ni(NO₃)₂ ·.6H₂ O were dissolved in water atan equivalent molar ratio. Sintered alumina particles were made with thesame process as in Example 4, except that an amount of the solutionequivalent to the amount of NiO·TiO₂ in the NiO·TiO₂ fine powder slurryin Example 4, was added as NiO·TiO₃. Although there was nonuniformity inthe properties of the particles, the particles showed, on average,approximately the same characteristics as those obtained in Example 4.Table 1 shows the characteristics of abrasive grains obtained inExamples 1 through 8.

                  TABLE 1                                                         ______________________________________                                                           Density  Crystal size                                                                           Hv (500)                                 Example                                                                              Seed        (g/cm.sup.3)                                                                           (microns)                                                                              (kg/mm.sup.2)                            ______________________________________                                        1      Ti.sub.2 O.sub.3                                                                          3.96     0.4      1920                                     2      Ti.sub.2 O.sub.3 2.8%                                                                     3.96     0.3      2000                                            solid solution                                                                alumina                                                                3      MgO.TiO.sub.2                                                                             3.96     0.5      2080                                     4      NiO.TiO.sub.2                                                                             3.95      0.23    2260                                     5      FeO.TiO.sub.2                                                                             3.97     0.5      2210                                     6      MnO.TiO.sub.2                                                                             3.96     0.5      2120                                     7      Ti(OH).sub.4                                                                              3.96     0.4      1950                                     8      NiO.TiO.sub.2                                                                             3.96     0.5      2210                                     ______________________________________                                    

Note: Crystal size according to SEM photographs.

Hv (500): Vickers hardness at a load of 500 g.

Particle size of abrasive grain: #24

EXAMPLE 9

An alpha-Al₂ O₃ abrasive grain was made using the same process used forNiO·TiO₂ in Example 4, except that 470 g of marketed CoO was used as acobalt source and sintering for seed synthesis was carried out in anargon atmosphere. The characteristics of the grain are shown in Table 2.

EXAMPLE 10

Using commercial-grade V₂ O₅ as a raw material, V₂ O₃ was obtained byheating a hydrogen atmosphere at 800° C. for one hour. An alpha-Al₂ O₃abrasive grain was made using the same process as in Example 1. Thecharacteristics of the grain are shown in Table 2.

EXAMPLE 11

Commercial-grade Ga₂ O₃ was pulverized in a pot mill made of zirconia,classified according to size using water as a classifying medium, and aslurry of less than 0.1 microns was obtained. An alpha-Al₂ O₃ abrasivegrain was made using the same process as in Example 1. Thecharacteristics of the grain are shown in Table 2.

EXAMPLE 12

An alpha-Al₂ O₃ abrasive was made using the same process as in Example11, commercial-grade R₂ O₃ was used. The characteristics of the grainare shown in Table 2.

EXAMPLE 13

An alpha-Al₂ O₃ abrasive grain was made using the same process as inExample 4, except that 510 g commercial-grade ZnO was used as a zincsource. The characteristics of the grain are shown in Table 2.

                  TABLE 2                                                         ______________________________________                                                          Density   Crystal size                                                                           Hv (500)                                 Example                                                                              Seed       (g/cm.sup.3)                                                                            (microns)                                                                              (kg/mm.sup.2)                            ______________________________________                                         9     CoO.TiO.sub.2                                                                            3.96      0.4      2000                                     10     V.sub.2 O.sub.3                                                                          3.96      0.5      2000                                     11     Ga.sub.2 O.sub.3                                                                         3.95      0.5      1900                                     12     Rh.sub.2 O.sub.3                                                                         3.96      0.5      1900                                     13     ZnO.TiO.sub.2                                                                            3.96      0.4      2100                                     ______________________________________                                    

Note: See Notes of Table 1

REFERENCE EXAMPLE 1 (Abrasive grain described in Japanese patentapplication 56-32369)

An alumina abrasive grain containing MgO (particle size #60) was madeusing the same process as in Example 21, which is described in Japanesepatent application 56-32369. The abrasive grain has the followingcharacteristics:

MgO content: 6%

Hardness Hv(500): 1410 kg/mm² Density: 3.61 g/cm²

Crystal size: 1 to 3 microns

REFERENCE EXAMPLE 2 (Abrasive grain described in Japanese patentapplication JP-A-60-231462)

Two hundred grams of pseudo-boehmite of SB Pural Alumina (marketed byCondea) and 1.5 liters of water were mixed in a beaker. Next, to producea sol, 0.30 liters of HNO₃ (3.6% by weight) were added to yield asolution having a pH of 2. Then, 7 kg of alumina balls and 1.5 liters ofwater were put into a 7.3 liter alumina pot which was rotated for 96hours so that the alumina balls could grind themselves. As a result, aslurry was obtained containing particles from the alumina balls producedduring the grinding operation. The specific surface area of theseparticles was 75 m² /g.

Next, the slurry was added to the alumina sol in an amount such that theparticles constituted 1.5% by weight of the alumina (Al₂ O₃). After twohours of stirring, the sol was transferred to a vat, and dried first at80° C. for 48 hours, then at 120° C. for 24 hours. After drying, the drygel was pulverized in a mortar and sieved to a maximum particle size of500 microns and a minimum size of 350 microns. The sieved dry gel wastreated at 750° C. for one hour to remove the NO_(x) contained in thenitric acid, and then sintered in a rotary kiln at 1400° C. for oneminute. The time taken to raise the temperature to 1400° C. was 15minutes.

The particle size of the sintered abrasive grain was 0.2 to 0.5 microns,with an average size of 0.3 microns; the Vickers hardness at a load of500 g was 2230 kg/mm² ; and the density was 3.89 g/cm³, or 98% of thetheoretical density.

REFERENCE EXAMPLE 3 (Abrasive powder described in Japanese patentapplication JP-A-61-254685)

The abrasive powder was made by the method described in Japanese patentapplication JP-A-61-254685. The abrasive grain had the followingcharacteristics:

Density: 3.92 g/cm³, Crystal size: 0.5 microns

Hardness HV (500): 2120 kg/mm²

EXAMPLE 14

Thirteen parts of borosilicate frit (a vitrified grindstone bond), twoparts of dextrin, and 2.5 parts of water were mixed with 100 parts ofabrasive grain #60 in Example 4. The borosilicate frit used was composedof 70% SiO₂, 7% Al₂ O₃, 18% B₂ O₃, 4.0% Na₂ O·K₂ O, and 0.5% CaO·MgO.After mixing, the material was formed in a press. The formed material,comprised of 45% abrasive grain, was dried at 110° C. for 20 hours. Thematerial was cooled, with especially slow cooling through 500° C. and600° C.

Thus, a vitrified grinding wheel was produced, with a bonding grade of Kas specified in JIS R6210. The size of the grains in the grinding wheelof this Example, corresponding to the abrasive grain in Example 4, was200×19×76.2 mm.

REFERENCE EXAMPLES 4 AND 5

In relation to the #60 abrasive grain in Example 2 and the #60 singlecrystal alumina abrasive grain 32A made by Norton, Inc., vitrifiedgrinding wheels measuring 200×19×76.2 mm were made using the sameprocess as in Example 14, and are named as reference samples.

EXAMPLE 15

The performances of the vitrified grinding wheels in Example 14 andReference samples 4 and 5 were evaluated under the following testconditions:

                  TABLE 3                                                         ______________________________________                                        Test Conditions                                                               ______________________________________                                        Machine        Okamoto Heiken, CFG-52 (3.7 kw)                                Grinding system                                                                              Plunge grinding, down cut,                                                    manual cut,                                                    Grinding material                                                                            SKD-1 (HRC 60)                                                                .sup.L 100 × .sup.h 50 × .sup.t 10                 Grinding Wheel 2000 m/min                                                     surface speed                                                                 Table speed    20 m/min                                                       Cut-in size:   Δ.sup.R 20 micron/pass                                   Total cut-in size                                                                            5 mm                                                           Grinding width 10 mm                                                          Spark-out      1 pass                                                         Grinding oil   Dry type (no oil used)                                         Dressing condition:                                                                          Single stone diamond dresser                                   Cut-in size:   .sup.R 20 micron/pass                                          Total cut-in size:                                                                           Δ0.2 mm/r.o.w.                                           Spark-out:     None                                                           ______________________________________                                    

Table 4 shows values for the grinding ratio, maximum electric powerconsumption (value excluding no-load power(0.4 kW)), and surfaceroughness.

                  TABLE 4                                                         ______________________________________                                                                          Surface                                                  Grinding Maximum     roughness                                                ratio    Consumption (M.sub.R Z)                                              (mm.sup.3 /                                                                            power       l = 2.5 mm                                  Sample No.   mm.sup.3)                                                                              (KW/cm)     N = 3                                       ______________________________________                                        Example 14   47       1.1         15                                          Reference Example 4                                                                        35       1.3         18                                          Reference Example 5                                                                         7       1.7         20                                          ______________________________________                                    

As Table 4 shows, the abrasive grain of the present invention has sixtimes the grinding ratio of References Example 5 (the single crystalalumina abrasive grain 32A that is available on the market) and hasremarkably higher values than Reference Example 4 (the grinding wheelusing a trace grade abrasive grain as described in Japanese patentapplication JP-A-60-231462). The grinding performance is greatlyimproved compared to conventional products. In spite of the improvedgrinding performance, the maximum power consumption is lower thanReference Examples 4 and 5, and the surface roughness is several gradesbetter than the Reference Examples. In addition, the burn of work piece,which occurs very frequently in Reference Example 5 and relativelyfrequently in Reference Example 4, did not occur at all in Example 14.The product of the present invention is an abrasive grain and a grindingwheel unsurpassed by any conventional product.

EXAMPLE 16

The parts of resorcinol were dissolved into ten parts of ethanol, thenadded to 100 parts of the abrasive grain of Example 14. This mixture wasdried at 100° C. for one hour to remove ethanol by evaporation.

The phenol resin bond BRL-2867 (about 70 wt. % solid, made by ShowaKobunshi Co.) was evenly coated on a compressed non-woven fabric basematerial at a rate of 100 g/m², and then the coated grinding materialswere spread out and excess material removed. 250 g/m² of grindingmaterial were deposited on the base material. The coated cloth was driedat 80° C. for 4 hours. Subsequently, the bond was coated evenly at arate of 200 g/m²,then dried at 80° C. for four hours. The temperaturewas then raised from 80° C. to 135° C. in two hours, retained at 135° C.for 30 minutes, and grinding cloths were thus obtained.

REFERENCE EXAMPLES 6 AND 7

Using the abrasive grain in Comparisons 1 and 2, grinding cloths usingthe non-woven fabric base material were made using the same process asin Example 16, and are named Reference Examples 6 and 7.

EXAMPLE 17

The grinding cloths in Example 16 and Reference Examples 6 and 7 werecut into 180 mm in diameter discs, then used as dry grinders under thefollowing test conditions.

                  TABLE 5                                                         ______________________________________                                        Test conditions                                                               ______________________________________                                        Sander:      Hitachi PHD-180C                                                 Grinding time:                                                                             1 min. × 10 grindings                                      Ground material:                                                                           (a) SPC        10 × 250 × t                                       (b) SUS 304     9 × 250 × t                          Load:        3 lbs.                                                           ______________________________________                                    

The grinding values are shown in Tables 6 and 7.

                  TABLE 6                                                         ______________________________________                                        Grinding SPC                                                                               Initial                Total                                                  grinding  9 to 10 minutes                                                                            grinding                                               amount    grinding amount                                                                            amount                                    Sample No.   (g/min)   (g/min)      (g)                                       ______________________________________                                        Example 16   22.3      5.2          128.0                                     Reference Example 6                                                                        15.5      2.1          67.9                                      Reference Example 7                                                                        19.0      3.3          96.1                                      ______________________________________                                    

                  TABLE 7                                                         ______________________________________                                        Grinding SUS 304                                                                           Initial                Total                                                  grinding  9 to 10 minutes                                                                            grinding                                               amount    grinding amount                                                                            amount                                    Sample No.   (g/min)   (g/min)      (g)                                       ______________________________________                                        Example 16   6.1       2.5          33.7                                      Reference Example 6                                                                        3.6       --           5.7                                       Reference Example 7                                                                        4.9       --           6.2                                       ______________________________________                                    

The reason Table 7 lacks figures for 9 to 10 minutes grinding amount inReference Examples 6 and 7 is that the ground materials started a burnin four minutes, the ground amount had fallen to an extremely low value,and the grinding therefore had to be stopped.

As Table 6 shows, when the ground material was cold-rolled steel SPC,the performance of the grinding cloth of the present invention was 1.3to 1.9 times the total grinding amount of Reference Examples 6 and 7 (agrinding cloth using trace grade abrasive grain as described in Japanesepatent application 60-231462). When the ground material was SUS 304, thetotal ground amount was 5.4 to 6.1 times that of the conventionalproducts--a remarkable improvement.

POSSIBILITIES FOR INDUSTRIAL USE

The abrasive grain of the present invention uses alpha-alumina, thecrystal size of which is finer than the sintered alumina abrasive grainconventionally used. Increasing the strength of the crystals enhancedthe strength of the abrasive grain, greatly improving its utility forheavy-duty or precision grinding.

Furthermore, the grinding wheel and coated cloth using the abrasivegrain of the present invention showed better grinding performance, thusgreatly contributing to its potential for industrial use.

What is claimed is:
 1. A process for manufacturing an alpha-Al₂ O₃abrasive grain comprising the steps of:adding particles of at least onecompound selected from the group consisting of Ti₂ O₃, MgO·TiO₂,FeO·TiO₂, NiO·TiO₂, CoO·TiO₂, MnO·TiO₂, ZnO·TiO₂, V₂ O₃, Ga₂ O₃, and Rh₂O₃, wherein the particles have an alpha-Al₂ O₃ structure and a size ofsubstantially less than 2 microns, into an alumina sol; gelling the sol;and sintering the gel at a temperature of 1000° C.-1400° C.
 2. A processfor manufacturing an alpha-Al₂ O₃ abrasive grain comprising the stepsof:adding particles of alpha-Al₂ O₃ solidly dissolved with at least onecompound selected from the group consisting of Ti₂ O₃, MgO·TiO₂,FeO·TiO₂, NiO·TiO₂, CoO·TiO₂, MnO·TiO₂, ZnO·TiO₂, V₂ O₃, Ga₂ O₃, and Rh₂O₃, wherein the particles have an alpha-Al₂ O₃ structure and a size ofsubstantially less than 2 microns, into an alumina sol; gelling the sol;and sintering the gel at a temperature of 1000° C.-1400° C.